Conventional methods for color thermal imaging such as thermal wax transfer printing and dye-diffusion thermal transfer typically involve the use of separate donor and receiver materials. The donor material typically has a colored image-forming material, or a color-forming imaging material, coated on a surface of a substrate and the image-forming material or the color-fonning imaging material is transferred thermally to the receiver material. In order to make multicolor images, a donor material with successive patches of differently-colored, or different color-forming, material may be used. In the case of printers having either interchangeable cassettes or more than one thermal head, different monochrome donor ribbons are utilized and multiple color separations are made and deposited successively above one another. The use of donor members with multiple different color patches or the use of multiple donor members increases the complexity and the cost of such printing systems. It would be simpler to have a single-sheet imaging member that has the entire multicolor imaging reagent system embodied therein.
There have been described in the prior art numerous attempts to achieve multicolor, direct thermal printing. For example, there are known two-color direct thermal systems in which formation of the first color is affected by formation of the second color. U.S. Pat. No. 3,895,173 describes a dichromatic thermal recording paper which includes two leuco dye systems, one of which requires a higher activation temperature than the other. The higher temperature leuco dye system cannot be activated without activating the lower temperature leuco dye system. There are known direct thermal imaging systems that utilize an imaging member having two color-forming layers coated on opposite surfaces of a transparent substrate. The imaging member is addressed by multiple printheads independently from each side of the imaging member. A thermal imaging system of this type is described in U.S. Pat. No. 4,956,251.
Thermal systems that exploit a combination of dye transfer imaging and direct thermal imaging are also known. In systems of this type, a donor element and a receiver element are in contact with one another. The receiver element is capable of accepting dye, which is transferred from the donor element, and also includes a direct thermal color-forming layer. Following a first pass by a thermal printhead during which dye is transferred from the donor element to the receiver element, the donor element is separated from the receiver and the receiver element is imaged a second time by a printhead to activate the direct thermal imaging material. This type of thermal system is described in U.S. Pat. No. 4,328,977. U.S. Pat. No. 5,284,816 describes a thermal imaging member that comprises a substrate having a direct thermal color-forming layer on one side and a receiver element for dye transfer on the other side.
There are also known thermal imaging systems that utilize imaging members having spatially separated regions comprising direct thermal color-forming compositions that form different colors. U.S. Pat. Nos. 5,618,063 and 5,644,352 describe thermal imaging systems in which different areas of a substrate are coated with formulations for forming two different colors. A similar bicolored material is described in U.S. Pat. No. 4,627,641.
Another known thermal imaging system is a leuco-dye-containing, direct thermal system in which information is created by activating the imaging material at one temperature and erased by heating the material to a different temperature. U.S. Pat. No. 5,663,115 describes a system in which a transition from a crystalline to an amorphous, or glass, phase is exploited to give a reversible color formation. Heating the imaging member to the melting point of a steroidal developer results in the formation of a colored amorphous phase while heating of this colored amorphous phase to a temperature lower than the crystalline melting point of the material causes recrystallization of the developer and erasure of the image.
There is also known a thermal system containing one decolorizable, leuco dye containing, color-forming layer and a second leuco dye containing layer capable of forming a different color. The first color-forming layer colorizes at a low temperature while the second layer colorizes at a higher temperature, at which temperature the decolorization of the first layer also takes place. In such systems, either one or the other color can be addressed at a particular point. U.S. Pat. No. 4,020,232 discloses formation of one color by a leuco dye/base mechanism and the other by a leuco dye/acid mechanism wherein the color formed by one mechanism is neutralized by the reagent used to form the other. Variations of this type of system are described in U.S. Pat. Nos. 620,204; 5,710,094; 5,876,898 and 5,885,926.
Direct thermal imaging systems are known in which more than one layer may be addressed independently, and in which the most sensitive color-forming layer overlies the other color-forming layers. Following formation of an image in the layer outermost from the film base, the layer is deactivated by exposure to light prior to forming images in the other, less sensitive, color-forming layers. Systems of this type are described in U.S. Pat. Nos. 4,250,511; 4,734,704; 4,833,488; 4,840,933; 4,965,166; 5,055,373; 5,729,274; and 5,916,680.
As the state of the thermal imaging art advances and efforts are made to provide new thermal imaging systems that can meet new performance requirements, and to reduce or eliminate some of the undesirable requirements of the known systems, it would be advantageous to have a multicolor thermal imaging system in which at least two different image-forming layers of a single imaging member can be addressed at least partially independently from the same surface by a single thermal printhead or by multiple thermal printheads so that each color can be printed alone or in selectable proportion with the other color(s).